1. Field of the Invention
The present invention relates to an active matrix liquid crystal display and, more particularly, to a miniaturized active matrix liquid crystal display having high reliability.
2. Description of the Related Art
An active matrix liquid crystal display uses a liquid crystal as a display medium. A pixel is disposed at each intersection in a matrix construction. Every pixel is equipped with a switching device. Information about the pixels is represented by turning on and off the switching devices. Three-terminal devices which are thin-film transistors having gate, source, and drain are most often used as the switching devices. The thin-film transistors are hereinafter often abbreviated as TFTs.
In the matrix construction, scanning lines (gate lines) extending parallel to a row are connected with the gate electrodes of the TFTs in this row. Signal lines (source lines) running parallel to a column are connected with the source (or drain) electrodes of the TFTs in this column. A circuit for driving the scanning lines and a circuit for driving the signal lines are also provided.
FIG. 2 shows one conventional active matrix liquid crystal display. A signal line driver circuit 202 for driving signal lines is mounted above the pixel matrix 201 of this active matrix liquid crystal display. A scanning line driver circuit 203 for driving scanning lines is disposed to the left. Indicated by 204 is a sealant material region.
FIG. 3 is a cross section of FIG. 2. As shown in FIG. 3 pixel TFTs 301 are covered by a liquid crystal material 302, which is held between a TFT substrate 303 and a counter substrate 304. On the other hand, signal line driver circuit and scanning line driver circuit are protected only by a thin film 306 of oxide or nitride. Indicated by 307 is a sealant material. Therefore, TFTs 305 forming these driver circuits are placed in a harsher environment than the pixel TFTs located inside the liquid crystal material.
In an attempt to solve the foregoing problems and to obtain long-term reliability, a display device structure having improved reliability has been devised. In this structure, both signal line driver circuit and scanning line driver circuit are placed within a liquid crystal material, as well as pixel TFTs.
FIG. 4 shows a known device of this improved structure. In this known structure, a sealant material or sealing material 403 is located outside both a signal line driver circuit 401 and a scanning line driver circuit 402. Therefore, the driver circuit TFTs are covered by the liquid crystal material, as well as the pixel TFTS. Furthermore, to miniaturize the liquid crystal display, three end surfaces (in FIG. 4, the top end surface, bottom end surface, and right end surface) of the counter substrate are made to conform to three end surfaces of the TFT substrate. Indicated by 404 is a pixel matrix.
These two conventional structures suffer from the following problems.
As shown in FIG. 5, in the conventional active matrix liquid crystal display, a short ring 501 is formed around the pixel matrix to protect the TFT devices from static charges. Since the signal lines 503 and scanning lines 504 connected with pixel TFTs 502 are all shorted, static charges produced during manufacturing steps, especially during rubbing steps, are prevented from being applied across the terminals of each pixel TFT 502. Indicated by 505 is a TFT substrate. Indicated by 506 are positions at which the device is cut by a laser beam.
In the first-mentioned conventional structure shown in FIGS. 2 and 3, it is common practice to cut the short ring together with glass substrates with a laser beam or the like in the final manufacturing step for the liquid crystal display.
However, in the second-mentioned conventional structure, in an attempt to minimize the size of the liquid crystal display, the counter substrate and the TFT substrate are preferably cut along common planes (in FIG. 4, the top end surface, bottom end surface, and right end surface of each substrate) from which no terminals are brought out. Accordingly, it is difficult to cut the short ring with a laser beam in the final step. In particular, the short ring is cut together with the substrates along a common plane. As shown in FIG. 6, after the cutting, the end surfaces of the substrates are exposed. If static charges are produced on the exposed end surface after the cutting, the internal pixel TFTs will be destroyed, thus making the display device defective. Indicated by 507 is the counter substrate. Indicated by 508 is a sealant material. Indicated by 503 is a liquid crystal material. Indicated by 504 is a bus line. Indicated by 505 is the TFT substrate. Indicated by 506 are the exposed end surfaces.